The present invention relates generally to charging an energy storage system, and more particularly but not exclusively, to low temperature fast charging of a lithium ion battery cell pack.
Many high-performance energy storage solutions now employ series-connected modules that, in turn, are series and parallel combinations of individual battery cells. Battery packs used with electric vehicles store large amounts of energy in a small space, producing high energy densities. The energy is converted into mechanical energy by the power train to move the vehicle, among other uses.
Low temperature ambient can affect performance of an electric vehicle for several reasons including an adverse effect on the battery cells (e.g., reduction in current carrying capacity for charging and discharging). One such effect is that fast charging at low temperature can, depending upon cell chemistry, result in lithium-plating inside the battery cell. Lithium-plating typically results in an irreversible capacity loss and is avoided whenever possible, particularly for high performance battery packs that can incur significant costs to replace/repair. Different cell manufacturers define “low temperature” differently, dependent upon cell chemistry and application. Low temperature as used herein means less than twenty-five degrees Celsius, and more specifically less than ten degrees Celsius.
In conventional systems where there is risk of lithium-plating, fast charging is disabled for low temperature cells. This can be a disadvantage to the user for several reasons, as well as disadvantageous to the electric vehicle community at large. It is a disadvantage to the user because instances in which a fast charge is necessary or desirable can become easily associated with low temperature ambient conditions. Without fast charge, the user may need to wait until the temperature of the cells rises or initiate slow charging (which can be quite slow, on the order of 0.01C). Availability of low temperature fast charging would allow the user to avoid the consequences of no/slow charging.
It is a disadvantage to the electric vehicle community because public charging stations are often open-air facilities able to serve a particular quantity of vehicles at one time. For users of vehicles in excess of this quantity, waiting in low temperature ambient conditions for an opportunity to begin charging can cause the temperature of the battery cells in the waiting vehicles to fall below a point where fast charging is available. Thus, the users must wait for the cell temperatures to rise, or initiate charging at a slow charging rate. A vehicle using a public charging station in slow-charge mode disadvantages other users, who may have to wait longer. The longer that they wait, they may also find themselves in the same situation where fast charging may become unavailable because the cell temperature in their vehicle has also fallen. Availability of fast charging in such a scenario would improve efficiency and throughput of public charging stations.
What is needed is an apparatus and method for fast charge preparation of low temperature cells potentially at risk of damage from fast charging.